Rudolf Weigand, 'The Development of the Glossa ordinaria to Gratian's Decretum', in The History of Medieval Canon Law in the Classical Period, 1140-1234: From Gratian to the Decretals of Pope Gregory IX, ed. by Wilfried Hartmann and Kenneth Pennington (Washington: Catholic University of America Press, 2008), pp. 55-97 (pp. 64, 72).